lkml.org 
[lkml]   [2006]   [Jul]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch] spinlocks: remove 'volatile'
From
Date
On Thu, 2006-07-06 at 13:33 -0600, Chris Friesen wrote:
> Linus Torvalds wrote:
>
> > On Thu, 6 Jul 2006, Mark Lord wrote:
>
> >> A volatile declaration may be used to describe an object corresponding
> >> to a memory-mapped input/output port or an object accessed by an
> >> aysnchronously interrupting function. Actions on objects so declared
> >> shall not be "optimized out" by an implementation or reordered except
> >> as permitted by the rules for evaluating expressions.
> >
> >
> > Note that the "reordered" is totally pointless.
> >
> > The _hardware_ will re-order accesses. Which is the whole point.
> > "volatile" is basically never sufficient in itself.
>
> The "reordered" thing really only matters on SMP machines, no? In which
> case (for userspace) the locking mechanisms (mutexes, etc.) should do
> The Right Thing to ensure visibility between cpus.
>
> The C standard requires the use of volatile for signal handlers and setjmp.
>
> For userspace at least the whole discussion of "barriers" is sort of
> moot--there are no memory barriers defined in the C language, which
> makes it kind of hard to write portable code that uses them.

You're falling into RBJ's trap. I did not say *MEMORY BARRIER*. While
for some uses of "volatile" that is the right substitute, for others it
is *optimization barrier* which matters.


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-07-06 21:41    [W:0.319 / U:0.372 seconds]
©2003-2011 Jasper Spaans. Advertise on this site